Mobile devices have not only become ubiquitous, but also their processing abilities and demands placed on the resources in the mobile device have increased with each new generation of technology. For instance, though lithium ion (Li-Ion) batteries reduce form factor and provide power for many mobile devices, demands for higher power on the mobile device have grown to the point that sufficient power cannot be achieved from the limited voltage supplied by Li-Ion batteries. One example that can drive power requirements of the mobile device is output volume of a power amplifier, such as for a loudspeaker or headphone connected to or comprising the mobile device.
To meet power requirements for a mobile device, often a boost circuit is used to boost a supply level provided from a battery to a higher level, and use the boosted signal as the supply voltage for the power amplifier. To improve efficiency, the output of the boost circuit is varied as a function of the output signal of the power amplifier, such as via envelope tracking in a class-H fashion. Furthermore, to increase signal swing, a common mode reference signal provided to the power amplifier is often set to half of the boost circuit output. Consequently, the common mode signal is also dependent on the output signal of the power amplifier.
Common mode signals of a differential power amplifier are converted to differential mode signals (CM-DM) due to mismatches in the power amplifier circuit. For example, mismatches occur in resistors connected to the power amplifier due to process variations, such as between input resistors and between feedback resistors connected to the differential inputs of the power amplifier. Any mismatch results in different currents flowing at the positive and negative inputs to the power amplifier, which are amplified by the power amplifier, causing CM-DM conversion. This CM-DM conversion increases distortion, including total harmonic distortion (THD), particularly second harmonic distortion, and degrades power supply rejection ratio (PSRR). These adverse effects can be exacerbated due to the common mode signal's dependence on the output signal of the power amplifier.
Accordingly, traditional methods attempt to reduce CM-DM induced-distortion in the power amplifier by using trimming resistors of the power amplifier to reduce mismatches. For instance, trimming resistors can be placed in parallel or in series with resistors connected to the power amplifier in an attempt to match effective resistances caused by the combinations of the trimming resistors and the resistors connected to the power amplifier. However, the addition of trimming resistors can be costly in terms of increased area and manufacturing time to adjust the trimming. Furthermore, matching the effective resistances can be difficult, especially when the trimming resistors are small in size. This difficulty can result in further mismatch that induces CM-DM distortion, even though trimming resistors have been added.